Many communication systems use automatic gain control to adjust various amplifier gains in the system. If the gain is too low, the system may be too sensitive to noise, while if the gain is too high the power consumption may be too great. Automatic gain controls seek to adjust the gains to balance these competing requirements. Automatic gain controls are a direct feedback mechanism, a mechanism that determines the difference between an output signal and its input signal (or a reference signal level) within a few operational blocks of a system. The automatic gain control applies a simple, linear function of this difference to the input, attempting to maintain the output at a predetermined ideal value. For ill-behaved or inconsistent inputs, or inputs that become non-linear (due to noise spikes, etc.), the feedback mechanism is often too rigid in its simplicity, and too direct in its parametric identity, to keep the system from transient instability.
In the receiver of a control system, the signals are subject to noise interference. A particular disadvantage of the direct feedback method is that is seeks to control gain even if it is responding only to the noise in the input. This is the case even when the envelope or power of the signal is monitored rather that the signal itself. A further disadvantage is that there may be no simple relationship between the level of the gain and the overall performance of communication receiver, which means that it is very difficult to know how to adjust the gain for optimal performance.